First of all we need to make sure we do not confuse Existentialism with Existence in the sense that I use the term which is as what is not Being. In other words there is a school of Existentialism but it is an argument between philosophers within Being, and then there is Existence which is what is beyond Being, i.e. what merely exists, and the Existentialists do not even try to go there.

I know this is very confusing. But it is signified by the fact you have asked what IS Existentialism, in other words we have not escaped from Being with your question and it is a specific school which uses the fact we have the word existence in our language. The reason we have it in our language is that it comes from the Arabs, who when they read Aristotle saw that he meant more than existence (Wajud) when he talked about Being, so they named that excess Kun (making) and when that was translated into Latin it became Existence, meaning what stands beyond Being. Only Indo-Europeans have Being in their languages so it is really only relevant to the Western tradition, but because we took over everything through colonialization it became relevant to others who speak languages without Being in them, thus truly existential languages.

Now this word existence lay dominant in our philosophical vocabulary until Kierkegaard and then Nietzsche, and then Heidegger and Sartre started taking a position which placed existence prior to essence, meaning the core of Being that just means that something is there. The way I like to talk about it is that Existence is neither aspect nor anti-aspect, where aspects are Truth, Reality, Presence and Identity. It turns out that when you go up the meta-levels of Being that at the fifth meta-level you hit a phase transition from Being to Existence. It can appear as Ultra Being, Emptiness or Void. If we think about Existence as either Emptiness or Void it is nondual, but if we think about it as Ultra Being then it is a singularity of absurdity or impossibility which just exists but we are seeing it from the outside rather than being encompassed by it.

It is really Jaspers that starts using the terminology of Existence and Heidegger adopts it, then we start seeing that this was the point of view of Kierkegaard and Dostoevsky previously. Heidegger was taken up by the French and a good book to read about that is Generation Existential by Ethan Kleinberg. After the French found out that Heidegger was associated with the Nazis they began to backpedal from this whole hearted adoption after the war. Except for Gadamer and perhaps Merleau-Ponty most of the French only had a limited Idea about what Heidegger was talking about. Existentialism was championed by Sartre and Camus who fell out with each other. And now it is seen as passé. This is mainly because Sartre it was decided did not understand Heidegger and of course Heidegger wrote a Letter on Humanism to disown Sartre’s interpretation. Since Heidegger is a more profound philosopher than Sartre or Camus everyone went back to the drawing board to try to understand Heidegger. Contemporary Philosophy starts with Heidegger. Nazi or not we cannot ignore him. Everything after Heidegger has to be influenced by him even Analytical Philosophy stands against Heidegger from the beginning with Carnaps rejection of his metaphysical nonsense. Even it it is seen as nonsense there has to be a position in relation to Heidegger. I am reading Cassirer Third Volume of Symbolic Forms and he references Heidegger.

So if one is going to understand what Existentialism means we need to start with Heidegger. Heidegger makes a very simple argument for using the Existential terminology of Jaspers. That is if we are going to get to a place prior to the arising of the subject/object dichotomy, i.e. to get to Dasein, and Dasein is the projection mechanism by which we project the world and then find ourselves in it, then we have to distinguish dasein and its categories from the categories of the objects within the world, and so we will call the categories of Dasein existentials. Heidegger was very reluctant to adopt the terminology of Existentialism from Jaspers but in the final draft of Being and Time he made that change suddenly. And when we get to the heart of Dasein where the Existentials overlap called Care, or Sorge, then there Heidegger imports everything from Kierkegaard’s idea of absurdist subjectivity. So Heidegger adopts Jaspers terminology and then he adopts Kierkegaard’s authentic subject as the core of Dasein. The term Care he gets from Augustine.

So what are these Existentials:

Befindlichkeit (foundness)

Rede (talk, discourse)

Verstehen (Understanding)

The opposites of all these are Fallenness which combines curiosity, ambiguity, idle-talk.

Turns out that Foundness is related to Thrownness which is related to the Past;

Understanding is related to Projection which is related to the Future, and seemingly incomprehensibly Fallingness is related to the Present. Discourse is not related to any moment of time which is unexpected. There are other Existentials like the They.

There is also the distinction of Ontological Difference between Being and beings, and the corresponding difference with respect to Dasein is Existence and Existentiells. The only Existentiell that Heidegger really talks about is Death which is the avenue to authenticity.

So from this perspective Existentialism is a way to understand human existence as a human situation in which we are interested specifically in the facticity of life beyond essences (ready-to-hand) and abstractions (present-at-hand) which are two modalities of being-along-side things in the world. When we are with others or alone with ourselves we are relating only to dasein at that is an existential relation rather than an ontological relation. Thus Being looks different from the human perspective of the one projecting the transcendences and who is encompassed by them. But this is an attempt to distance oneself from essentialist ways of looking at things, which is based as Heidegger says on Metaphysics as is ‘humanism’ as a historical approach that centers on Man rather than Being, Being is forgotten and instead we find ourselves lost among beings some of which we treat as objects even though they are other human beings.

Existentialism turned out to be somewhat of a dead-end as a Western philosophical movement. It did not escape from metaphysics and Heidegger abandoned it eventually for something he felt was deeper, i.e. Beying rather than Being in Contributions. Sartre moved on to other issues in his Critique of Dialectical reason which is his really great book no one reads. It is about how to treat the dialectic dialectically and to found it on fundamental revolutionary human relationships. Camus died in a car accident as did Merleau-Ponty. Probably the most genuine member of the Existentialist movement was Kierkegaard who embraced absurdity and paradox as the fundamental basis of subjectivity. The basic argument is that if you get rid of essence then you have gotten rid of the basis of meaning so existence is absurd or as Sartre would say that means you have to produce your own meaning in life. But all the thinkers that are lumped together under the rubric of Existentialism had very different philosophies. We should probably recognize Jaspers as the core thinker who brought the term to prominence. Heidegger found it convenient to appropriate this terminology and the work of Kierkegaard within Being and Time but then abandoned that tack. Sartre, Camus and others in France that thought of themselves as Existentialists were looking for something different from Bergson or French NeoKantianism and thus felt they found something interesting in Husserl and Heidegger’s work. They saw Heidegger’s work as compatible with their Marxist interpretations of Hegel. But later Heidegger repudiated them and they repudiated him due to his Nazism. So Existentialism of the interwar period and immediately after the WWII quickly fell apart as other Continental philosophers moved on to try to work out what Heidegger really meant. But because Heidegger did not publish Contributions that was impossible because he merely hinted at his real position which was as he thought the one to actually bring metaphysics to an end after Nietzsche the last metaphysician. But we have noticed that whoever says they have brought Metaphysics (essence and abstraction thinking) to an end actually ends up merely repeating the sins of metaphysics. Thus we get Postmodernism that lingers on.

But the actually most interesting thing about Continental Philosophy is that Merleau-Ponty goes on to discover along with Heidegger and Derrida Hyper Being, and then on his own Wild Being. Derrida explores Hyper Being as Difference (differing and deferring) and Deleuze goes on after that to explore Wild Being. These other modalities of being-in-the-world, ways of relating to non-dasein objects in the world are extremely interesting and turn out to give us the series of meta-levels of Being which we are still trying to come to terms with. Basically we are stuck trying to come to terms with Being and Time just like the French did in various waves before and after the Second World War. What Heidegger did was come up with an extremely subtle solutions to the problems of neo-Kantianism which was the pre-war status quo and this is seen in his courses leading up to Being and Time. The reason that Being and Time made such an impact was that Heidegger hand not published anything before that and only gave courses during which he tried to solve the problems of Husserl’s phenomenology and its relation to neo-Kantianism. Heidegger solved these problems in such a clever and subtle way that he unmasked a history of metaphysics that no one suspected existed prior to him that started with Aristotle. Heidegger as medievalist is going back to Aristotle and discovering that he is a phenomenologist par excellence. And so Being and Time attempts to take us back to the kinds of Knowledge that Aristotle pointed out to us which we had lost track of in our history. It turns out that these kinds of knowledge that Aristotle points out in his Ethics are isomorphic to the Divided Line of Plato. And so Heidegger is taking us back into the Core of the Western worldview and reminding us of what is there. Basically the modes of Being of Dasein relate to kinds of knowledge in Aristotle. Present-at-Hand (extant) is Episteme of Science, this is basically the Algebraic-geometrical reduction of the world discovered by Descartes but in the time of Aristotle and Plato was based on Geometry eventually written down by Euclid and which became the basis of reason within our tradition. Ready-to-hand is the Techne of Poesis which we have seem to have lost but that is found in our immersion in the technological infrastructure of the world. Finally below that is Phronesis (judgment) of praxis (action) which is a pragmatism taken from Emil Last who got it from C.S. Peirce. Heidegger tried to use Last’s NeoKantianism influenced by Husserl and Peirce to attempt to disclose the facticity of life below the level of essence, i.e. the ready-to-hand. Husserl’s big discovery was that essence perception was different from abstraction, and that is the basis for the difference between the modes of Being in Heidegger which are called by me Pure Being (Parmenides) and Process Being (Heraclitus). Heidegger is following Nietzsche and Hegel in attempting to emphasize Process Being over Pure Being. Hegel thinks that ultimately they are the same. Nietzsche says that Pure Being is an illusion and there is only Heracltian Flux which he sees as Will to Power. Heidegger however wanted to go beyond this perspective and attempt to make a formal indication of the facticity of life following in the footsteps of Dilthey and others that proposed philosophies of Life. Husserl does the same thing in Krisis which talks about the Lifeworld and its estrangement from Science. Dasein was Heidegger’s final formulation of the formal indication of life which did not disturb it but disclosed it. When Heidegger found this strand of thought in Aristotle as well, i.e. the emphasis on disclosure then he knew he could completely overturn (deconstruct) modern philosophy which had forgotten its roots in disclosure. Existentialist terminology was a way to make clear the difference between this disclosure in the human situation from everything else we layer on top of that like essences and abstractions. When Sartre and Camus latched on to the idea that Existence came before Essence they missed that essential point and were caught up with the terminology of Nothing being Nihiliated or Nullity Nullified, and so Sartre took off with that idea and actually made something very interesting of it, but that is just a small part of Heidegger’s attempt to explain the nature of Dasein and how to project Being it must go beyond Being as a whole into the Nothing and Nihilate it in order to project it as something that encompasses Dasein. This is the basic paradox of the Trinity where the Father creates the world in which the Son is ensconced as an incarnation which Kierkegaard labels an absurdity, but takes it up and makes it the core of his belief. Since Being itself is a contradiction, or even a paradox, or even an absurdity, if not an impossibility it is hard for Dasein not to embody this paradox and absurdity within itself. But actual existence not as Ultra Being but as Empty or Void is not absurd or paradoxical but is instead Supra-rational and that is why the existentialists never got beyond Being because if they had they would have recognized its non-duality. Heidegger was not interested in Sophia of Virtue or the Nous of the Numinous that Aristotle also talked about as kinds of knowledge because Heidegger saw them as a prioris and thus dead, not living and so he was not interested in them in the least and left them out of his return to Aristotle.